For a large variety of technical reasons and for quite different purposes, it may be required to detect the position of moving elements or parts of a component by use of measurement technology. As examples for the use of an inductive rotational angle sensor, there could be mentioned—in the field of automobiles—the detection of the position of the accelerator, the throttle valve and the steering wheel. The advantage of inductive sensor systems resides in the contactless detection of a path position or rotational position.
The invention is related specifically to inductive position sensors as schematically depicted by way of example in FIG. 1 and described, for instance, in WO-A-2004/072653 and WO-A-2003/067181. Shown in this Figure is a one-channel position sensor of the inductive type for use as a rotational angle sensor.
Said sensor 10 comprises two transmitter units 12,14 in the form of transmitter coils, each of the coils generating an electromagnetic alternating field with a site-dependent amplitude. The two transmitter units 12,14 are controlled by a control unit 16 as will be described further below.
Sensor 10 is further provided with a movable element 18 which in this example is formed as a rotary disk or another type of rotating element. Said element 18 comprises an oscillating circuit 20 including an inductance 22 and a capacitance 24. The element 18 and respectively the oscillating circuit 20 are provided to rotate within a total electromagnetic alternating field which is generated by the mutual overlap of the two electromagnetic alternating fields of both transmitter units 12,14. Depending on the rotational position, oscillating circuit 20 will produce an electromagnetic alternating field having the same frequency as the alternating fields of the two transmitter units 12,14, wherein the alternating field produced by oscillating circuit 20 is shifted in phase relative to the two other alternating fields. The degree of the phase shift is a measure of the present rotational position of element 18. The signal of oscillating circuit 20 is received by a receiving unit 26 formed as a receiver coil, and the received signal is processed in an analysis unit 28; particularly, the phase position of this signal relative to the signals fed to the transmitter units 12,14 will be determined.
The transmitter coils and respectively transmitter units 12,14 will modulate low-frequency oscillations of the same frequency onto a carrier signal which is identical for both transmitter coils. The modulated oscillations of the two transmitter coils are phase-shifted by 90° relative to each other. Both trans-mission signals will energize the LC oscillating circuit 20. The strength of the excitation is proportionate to the coupled inductivity between the respective transmitter coils and the oscillating-circuit coil (inductance 22). Depending on the position of the movable element 18, each transmission signal will be coupled with a different strength into oscillating circuit 20. Within oscillating circuit 20, a modulated oscillation is generated which has the same frequency as the transmitted modulation signal. Relative to the transmitted modulation, the modulated oscillation of oscillating circuit 20 will have a phase shift which is dependent on the amplitude ratio of the modulation signals—coupled into oscillating circuit 20—of the transmitter coils. The signal generated in oscillating circuit 20 is passed on to the receiver coil (receiver unit 26), as already mentioned above.
The mathematical approach on which the measuring principle is based, can be outlined as follows. When two sinusoidal oscillations of the same frequency which are phase-shifted by 90° relative to each other, are added to each other, a sinusoidal oscillation of the same frequency will be generated. The phase shift of the generated oscillation is a function of the amplitude ratio between the two added oscillations.
For many uses, a two-channel inductive position sensor will be required. In such a sensor, the sensor arrangement shown in FIG. 1 is provided twice. There is no separation between the two sensor arrangements; both channels are coupled into each other. Such a two-channel inductive sensor is shown e.g. in FIG. 2. In FIG. 2, those elements of the second channel which are identical to the elements of the one-channel sensor according to FIG. 1, are marked by the same reference numerals but provided with a prime. A further two-channel inductive sensor is described e.g. in US-A-2002/0179339.
The known inductive sensors of the above design have been basically found useful in practice. However, for certain uses, the current consumption of such sensors is occasionally too high. Further, since no sine or cosine signals are used at the input side, which is of advantage for an effective use of the system, a quite considerable post-processing expenditure in the form of filtration processes and the like will be necessitated at the output side, which is also not desirable and will increase the space requirement on an ASIC.
Known from US-A-2005/0030010 is an inductive position sensor of the type mentioned and described above which is operated with a PWM signal as a modulation signal. Also this sensor requires an increased expenditure for signal post-processing, rendering the overall arrangement more complicated. For the filtration of the modulation signal so as to obtain the sine or cosine development, a low-pass filter with relatively low limiting frequency will be required, which entails the need for additional circuit components and thus causes an increased space requirement in the ASIC.